Radiotransmitting system



Aug. 16,, 1932.

V. E. TROUANT RADIOTRANSMITTING SYSTEM Filed. Oct. 8, 1927 Jhllh 1: mm M g. hm on N mm 2823 -EE h wN om MN 3 .555 QB MA mm NR hm, T 1 mm R l w p Y. mm IHI 1 J D |\HM.| 3 Q t m m w kmmkug Q WW m LoSo \QKRRET \Emkmmak km. E. mm m im u a 1 Q fi M U) mt ifi mkt fiwkg LQ Q 0 7 w INVENTOR Virgil E "770 uam.

ATTORNEY Patented Aug. 16, 1932 UNITED STATES PATENT OFFICE VIRGIL E. TROUANT, OF WILKINSBURG, PENNSYLVANIA, ASSIGNOB TO WESTINGHOUSI ELECTRIC 8: MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA BADIOTRANSMITTING SYSTEM Application filed October 8, 1927. Serial No. 224,889.

My invention relates to radio transmitting systems, and it has particular relation to systems of the type in which the radiated power is maintained substantially constant while the frequency thereof is varied in order to transmit signals.

In the majority of radio-transmitting systems designed for telephonic communication, the signal is impressed on a constant-frequency carrier-wave as a modulation of the amplitude thereof. As an alternative, it has been proposed to alter the frequency of the carrier-wave in accordance with voice or other sounds and to thus transmit the signal as frequency-modulation rather than as amplitude modulation. Systems of the latter type have not given very satisfactory results in the past because of the many difliculties encountered when an attempt was made to shift the tuning of the frequency-determining circuits associated with the transmitting apparatus at a rate sufliciently rapid to follow the tones of the human voice or the notes of the musical scale.

In my copending application, Serial No. 199,677, filed June 18, 1927 I have disclosed certain methods whereby the frequency of radiated oscillatory-energy may be advantageously modulated by voice or other signals by directly varying the frequency of the oscillations generated by a piezo-electric crystal-controlled master-oscillator. The present invention is akin to the invention disclosed in my copending application and is in the nature of an improvement thereon.

One object of my invent-ion is to provide appropriate means and circuit connections whereby the carrier wave of a radio transmitting system may be modulated, as to 7 requency, in a smooth, continuous manrer, eliminating the abruptchanges incident to frequency-modulation systems known to the prior art.

Another object of my invention is to provide a modulating system that is equally effective for both voice and telegraphic signal transmission.

Another object of my invention is to provide a modulating system in which the frequency of a piezo-electric crystal-controlled master-oscillator may be varied in accordance with audio or other signals without the expenditure of any considerable amount of power.

Still another object of my invention is to provide a modulating system that shall be substantially without time-lag in its response to signal impulses impressed thereon, rendering it particularly useful in picture transmission and the like.

According to my invention, the frequency of the oscillations generated by a piezo-electric crystal-controlled master-oscillator is caused, by the action of certain agencies later to be described in detail, to shift above or below the natural frequency of the crystal proper, the amount of the frequency-shift being proportional to the amplitude of the signal impressed on the system, and the rate at which the shift takes place being controlled by the frequencies involved in the signal.

In general, piezo-electric crystal-controlled oscillation-generators comprise a thermionic device having an input, or grid, circuit including the c stal, and an oscillatory out put, or plate, clrcuit which is tuned to a frequency approximating the natural frequency of the crystal section. The thermionic device is maintained in the oscillating state by reason of the transfer of energy from the output :ircuit to the input circuit through the inter-electrode capacity of the device. The frequency of the oscillations is largely determined by the natural frequency of the crystal, the spacing of the sup orting electrodes therefrom, the constants of the grid, or input, circuit and, as I have discovered, by the constants of the output, or plate, circuit. In other words, the oscillation frequency can,

within definite limits, be forced to follow the tuning of the output circuit, provided the said circuit is not very greatly de-tuned in either direction from the state corresponding to the fixed tuning of the grid, or input circuit.

I am not prepared, at this time, to state the exact theory underlying my invention, but it is possible that the shift in oscillation freq'lency is caused by a change in the phase-of tne energy fed back to the input circuit from cies.

the output circuit of the device. As is well known to those skilled in the art, in order to maintain oscillations in a network comprisin a thermionic tube, at any given frequency, so ely by reason of energy feed-back through tube-capacity, it is necessary that the output circuit have an inductive reactance to currents at the frequency in question. Should the output circuit be capacitive in reactance, the phase of the feed-back is incorrect for the maintenance of oscillations, and it might be said that the system would, in that event, be degenerative rather than regenerative. For this reason, it is hi hly probable that the de-phasing of the fee' -bac energy, within certain small limits, is instrumental to some extent in altering, by small amounts, the oscillation frequency.

In a preferred embodiment of my invention, the slightshift in the tuning of the out-, put circuit of a thermionic oscillation generator is accomplished by connecting a reactlve network in shunt to the oscillatory, or frequency-determining, circuit comprised therein, and by altering the effective value of the network in response to signalling frequen- The grid circuit of the thermionic device preferably includes a piezo-electric crystal mechanically resonant to the desired mean oscillation frequency, though I have, instead, utilized tuning-fork control with surprisingly good results. The changes in the reactance of the network may, if desired, be restricted to variations above a certain normal value, or may be restricted to variations-below the normal value.

In order to effectively control the value of the reactance network associated with the outut circuit of the thermionic oscillator, I have ound it practical to connect the plate-filament path of a second thermionic device in shunt to a portion thereof, and to change the resistance of the path by impressing signalenergy on the grid of the device. In this manner, by suitably proportioning the permanent direct-current bias potential applied to the grid of the thermionic device, hereinafter termed the shifter-tube, the reactance changes in the network may be restricted to positive values, to negative values, or vto both in combination, depending upon the type of signal it is desired to transmit.

The novel features which I consider characteristic of my invention are set forth with particularity in the appended claims. The invention, in its generic aspect, however, both as to its organization and its method of operation, together with further objects and advantages thereof, will best be understood by reference to the following description, taken in connection with the accompanying draw The single fi re of the drawing is a schematic view of t e circuits and apparatus of a' radio transmitting system comprising a preferred embodiment of my invention.

Referring specifically to the drawing, a thermionic oscillator device 1, having the usual filament 2, plate 3 and grid 4, is provided with a grid, or input, circuit comprising a piezo-electric crystal section 5 supported between electrodes 6'and 7 which are connected to the grid and filament, respectively. A choke coil 8 and a biasing battery 10 are connected across the electrodes and together serve to maintain the grid at the best operating potential.

The plate, or output, circuit, comprises an inductor 11 having a variable condenser 12 connected in shunt thereto, and includes a source 13 of plate potential. A by-pass condenser 14 is connected across the plate-poten-- tial source to provide a path for the radiofrequency component of the plate current.

Connections 15 and 16 extend from appropriate points on the inductor 11 to a frequency-changing device 17 which is arranged to select and amplify an ap ropriate harmonic of the natural crystal-fi'equency, and the amplified harmonic frequency 1s impressed on the in ut circuit of an amplifying device 18 tuned t ereto.

The output of the amplifyin device is impressed on the input circuit 0 a power-amplifier 20, preferably-comprising a plurality of high-power, water-cooled thermionic devices (not shown), from which energy at the harmonic-freqency is normally supplied through a variable coupling device 21, .to a

radiating structure comprising an antenna 22, a variable reactor 23 and a counterpoise 24.

They counterpoise may be replaced by a ground connection, if desired, inwhich event,

the coupling reactor could advantageously be replaced by a coupling transformer having its primary winding included in the output circuit of the power amplifier and its secondary winding included serially in the antennaground structure as shown in the Reissue Patcut to Heising 14,967.

The frequency changer 17 is not an essential element of the system, it being feasible to directly amplify and radiate the frequency generated by the crystal-controlled oscillation generator. It has been found, however,

that crystal sections so dimensioned-as tooscillate at frequencies corresponding to the extremely short wave-lengths now being utilized for long distance transmissions are quite prone to breakage during use. For that reason, it is preferably to utilize a crystal oscillating at a low frequency and to select, for transmission, one of the higher harmonics of the oscillation frequency.

The fundamental frequency generated by the local oscillator may be conveyed by radio or over wire lines to additional radio transmitters and there directly amplified and radiated, or there passed through appropriate frequency changers before amplification and radiation. In addition, harmonic freuencies other than the frequency utilized or radiation from the station at which the illustrated apparatus is situated, may also be conveyed to distantly located radio transmitting stations and there either amplified and radiated or changed in frequency before radiation. In either event, the modulation impressed 'on the fundamental frequency, in the manner later to be described in detail, appears as a modulation of any harmonic utilized fortransmission or radiation, being represented by a shifting of the frequency of said harmonic toward and away from the normal frequency thereof. In order that the oscillations generated by the oscillator device 1 may be modulated by signal-frequencies, I provide a plurality of shifter-tubes, 25 and 26, each of which may be a thermionic device of the usual type having a filament 27, a grid 28 and a plate 30. Similar elements of the shifter-tubes are connected in parallel, and the filaments thereof are supplied with power from a common source 31. If desirable, the plurality of tubes illustrated may be replaced by a single tube capable of handling an equivalent amount of power.

The commoninput circuit of the shiftertubes comprises the secondary 32 of an audiofrequency transformer 33, the primary 34 of which may be energized from a microphone 35 or may be included in the output circuit of an amplifying device (not shown).

The plate circuit of the shifter-tubes may be traced from the plates 30 over a conductor 36, through a portion of an inductor 37, through a portion of the plate-potential source 13, to a conductor 38 which is common to the filament circuits of the shifter-tubes and the oscillator tube 1. The end of the inductor 37 opposite the end which is connected to the voltage source 13 is coupled to the plate of the oscillator-tube through a stopping condenser 39, and a by-pass condenser 41 is connected in shunt to the portion of the voltage source 13 that supplies plate potential to the shifter-tubes.

The inductor 37 and the stopping condenser 39 accordingly constitute, a reactive network connected across a portion of the output-circuit of the oscillator-tube and together influence, to some extent, the tuning of the said circuit.

In the operation of a system arranged according to my invention, the output circuit of the oscillator-tube is preferably tuned to approximately the natural frequency of the piezo-electric crystal section, and oscillations are accordingly maintained therein by reason of the feeding-back of energy from the plate-circuit to the input circuit through the capacity existing between the plate and grid electrodes.

Any alteration in the constants of the out put circuit, which causes the tuning thereof to vary slightly from the aforementioned apprpximation, has a slight though very defmite effect upon the frequency of the oscillations generated. V

In the modification of my invention illustrated in the drawin the plate-filament paths of the shifter-tn s are included in a series circuit com rising a portion of the inductor 37, and t e resistance of the combined late-filament path is controlled at signal equency by potentials placed on the grids of the tubes by the action of the microphone 35 and the transformer 33. When the current through the portion of the inductor 37 referred to is varied at signal-frequency, the effective 'reactance of the network including the entire inductor 37 is altered, and the frequency of the generated oscillations is made to approach and recede from the mean, or normal, frequency at a' rate and in an amount proportional to the energization of the microphone.

The direction and average amount of the frequency-shift is controlled by properly adjusting the biasing potential applied to the grids of the shifter-tubes, with respect to the'potential of the signal frequencies applied to the grids thereof.

-The shift in frequency of the generated oscillations is never very large, but is amply sufficient to impress signaling frequencies thereon. In one commercial embodiment of my invention, which has been in successful operation over an extended period, the mean, or normal, oscillator frequency is 970 kilocycles and the shift obtained with voice modulation isof the order of 800 cycles.

The system-of modulation I have disclosed herein is not restricted to crystal-controlled oscillation-generators, but is also applicable to generators of other types, among which maybe mentioned the well-known Hartley circuit, wherein the frequency is determined by a tuned circuit, and capacitive or inductive means are employed for feed-back purposes. I have found that, if the inductor in the frequency-determining circuit is shunted by a network of the same character as that shown in the drawing, a much larger frequencyshift may be obtained than if the oscllation generator is of the crystal-controlled type, such shift being in some instances as large as 10,000 cycles away from the normal, or mean, frequency. 1

The apparatus involved in the amplification and radiation of the shifting frequencies originating in the master oscillator differs but slightly from similar apparatus utilized in radio systems of the types now familiar to those skilled in the art. It has been found beneficial in some instances, to slightly increase the decrement of the tuned coupling circuits employed between amplifier stages,

but there is no necessity for carefully biasing each thermionic amplifier device to the straight portion of its characteristic curve. Inasmuch as exact tuning of the coupling devices between the several amplifier stages is not found to be obligatory, the tendency toward self-oscillation 1n the several stages is noticeably absent.

My invention is also of great value when it is desired to control aradio-t'ransmitter from a remote point, inasmuch as all of the control energy required may be transmitted from a distant microphone over apair of relatively small wires.

Although illustrated as particularly adaptticularly applica le to picture-transmissionand television systems, wherein frequencyshifts of manythousands per second are necessary.

While I have shown and described only a single embodiment of my invention, it will be apparent to those skilled in the art that many modifications are possible. My invention, therefore, is not to be limited except insofar as is necessitated b the prior art and by the spirit of the appen ed claims.

I claim as m invention:

1. In an oscillation generator comprising a thermionic device having a tunable output circuit, a reactive network connected across said circuit, a circuit including a space discharge path conductively connected in shunt to a portion of said network, and-means for varying the conductivity of said path, whereby the frequency of the generated oscillations may be controlled.

2. In an oscillation generator comprising a thermionic device having a tunable output circuit, a reactive network connected across said circuit, a circuit including a space discharge path conductively connected in .shunt to a portion of said network, and vibrationresponsive means for varying the conductivity of said path, where y the frequency of the generated oscillations may be controlled by said vibration-responsive means.

3. In an oscillation generator comprising a thermionic device having a tunable output circuit, a reactive network connected across said circuit, a circuit including a space discharge path conductively connected in shunt to a portion of said network, a vibration-responsive device, a control-electrode in said ama path, and means actuated by said vibrationresponsive device for placing a variable electric charge on said control-electrode, whereby the conductivity of said path may be altered and the frequency of said generated oscillations controlled.

4. In combination with an electrical discharge device having an input circuit and an output circuit substantially without mutual induction, a piezoelectrical crystal means associated with said input circuit for determining approximately the frequency of oscillatory currents generated in said output circuit, tuning means for said output circuit, and signal-controlled means for varyin said tuning for varying said frequency.

n testimony whereof, I have hereunto subscribed my name this 6th day of October,

VIRGIL E. TROUANT. 

